1. Field of the Invention
This invention relates to a high level voltage generator, and more particularly to a high level voltage generator for rapidly pumping a high voltage node by simultaneously driving two high voltage pumping circuit when the potential level of the high voltage node is largely dropped.
2. Description of the Related Art
In general, a semiconductor memory device such as DRAM is a random access memory which reads/writes data in/from a memory cell including a transistor and a capacitor. The read or write operation of the DRAM will be described as follows. First, a row address strobe signal /RAS becomes in an active state and then a word line is activated by a row address signal. Then a column address strobe signal /CAS becomes in an active state and a bit line is activated by a column address to select a memory cell. The data is read/written in/from the selected memory cell.
The DRAM uses a high level voltage Vpp having a higher level than the power supply Vcc considering the voltage loss due to a threshold voltage Vtn of the MOS transistor constituting the memory cell in driving a word line. The high level voltage Vpp is generated from a high level voltage generator for a word line which generates a potential of power supply Vcc+threshold voltage Vtn+.DELTA.V.
FIG. 1 shows a conventional high level voltage generator. The high level voltage generator includes a high voltage level detection means 100 which detects a potential level of a high voltage node Vpp and compares the detected level with the target level to generate a pumping enable control signal ppe. The high level voltage generator includes a ring oscillator 200 for periodically generating a pulse signal by the pumping enable control signal ppe from the high voltage level detection means 100 in a power-up signal of active state and a high voltage pump means 300 for pumping the high voltage node to a high voltage level by the pulse signal from the ring oscillator 200. The high level voltage generator further includes a power-on precharging circuit (not shown) for initializing the potential level of the high voltage node Vpp to a constant voltage, before the high voltage pump means 300 operates.
The operation will be described as follows. First, in a power-up initial state, before the high voltage pump means 300 operates, if the potential level of the high voltage node Vpp is lower than an external voltage Vext, the power-on precharing circuit (not shown) initialize the high voltage node Vpp to a voltage which is lower than the external voltage Vext by a threshold voltage Vt of a MOS transistor. Thereafter, a power is supplied to a DRAM device, the substrate voltage pump circuit (not shown) operates and pumps the substrate voltage Vbb below a constant voltage. At this time, if the substrate voltage Vbb is below a constant voltage, the power-up signal is applied to the ring oscillator 200 to generate the pulse signal having a constant period.
The high voltage pump means 300 pumps the high voltage node Vpp to rise the potential level of the high voltage by the pulse signal from the ring oscillator 200. The high voltage level detector 100 detects whether the potential level of the high voltage node Vpp reaches to the target level. If the detected potential level of the high voltage node Vpp reaches to the target level, the high voltage level detection means 100 makes the ring oscillator 200 not to generate the pulse signal. Accordingly the high voltage pump means 300 does not more operate.
The high level voltage generator constantly establishes the pumping drivablity in the high voltage pump means 300. When the consumption of the high voltage is large, the pumping time becomes long. Otherwise, when the consumption of the high voltage is low, the pumping time becomes short. Therefore, so as to reduce the pumping time, if the high level voltage generator is designed to have the large drivability, the potential level of the high voltage node is rapidly recovered. However, during the required time that the high voltage level detection means 100 detects the potential level of the high voltage node Vpp to control the high voltage pump means 300, the high voltage pump means 300 continuously operates. Accordingly, the potential level of the high voltage node Vpp exceeds the initial target level.
Accordingly, so as to reduce the pumping time, if the prior high level voltage generator is designed to have the large drivablity in the high voltage pump means 300, the high voltage level is rapidly recovered. But the undesired current consumption becomes large and the noise caused. On the other hand, so as to reduce the current consumption and the noise, if the high level voltage generator is designed to have the low drivability in the high voltage pump means 300, the recovery time of the high voltage level becomes long and data in a memory cell becomes lost.